1. Technical Field
The present invention relates to a method of manufacturing a semiconductor device, in particular, a method of improving selection ratio of materials such as Si and Ge at the time of etching.
2. Related Art
The utility of a field effect transistor formed on a SOI substrate is brought to an attention because of easiness for isolating elements, latch up free and small source and drain contact capacitance. In particular, a perfect depletion SOI transistor has the capability of fast driven operation with low power consumption and is easily driven by low voltage, enhancing active research for driving SOI transistors with the prefect depletion mode. “Separation Bonding SI Islands (SBSI) for LSI application,” Second International SiGe Technology and Device meeting, Meeting Abstract, pp. 230 to 231, May 2004 written by T. Sakai et al. discloses a method (SBSI method) in which a SOL layer is formed on a bulk substrate to reduce a cost of manufacturing SOI transistors. In the SBSI method disclosed in this document, a hollow portion is formed between a Si layer and a Si substrate by forming Si and SiGe layers on a silicon substrate and selectively removing only the SiGe layer with utilizing different etching rate between Si and SiGe. Then silicon exposed in the hollow portion is thermally oxidized. The oxidized silicon layer is embedded to a space between the Si substrate and the Si layer, forming a BOX layer between the Si substrate and the Si layer. In this conventional SBSI method, however, the etching ratio of the SiGe layer to the Si layer was deteriorated if it takes long time to selectively etch only the SiGe layer among multiple layers such as Si, SiGe and Si layers. Such deterioration unintentionally accelerates etching of the Si layer at the time of removing the SiGe layer, facing difficulty in forming stable configuration and uniform thickness of large area's SOI layers or if it has various configurations, lowering the yield.